1. Radio Selected Damped Lyman  Systems

Jeremy Darling
(CASA, University of Colorado)
Outline:
1. Damped Lya Systems
2. Motivation
3. Intervening Absorption
4. Intrinsic Absorption
[5. H2CO Absorption]
6. Summary

2. Damped Lyman  Systems NHI ≥ 2  1020 cm-2
By definition,
For the 21 cm HI line,
In practice, we’ll refer to DLAs as any sufficient column, regardless of host or setting.
NHI ≥ 2  1020 cm-2
NHI = 1.8  1018 cm-2 (Tspin / )   d

3. DLAs: Motivation for 21 cm Searches
Optical spectroscopic selection:
Requires background UV source
Redshift into atmospheric window (z > 1.65)
Pencil beam (single los, single cloud)
DLA studies:
Damped/saturated line
(very important for EOR studies!)
Optical identification of DLAs
Molecular absorption:
Extremely rare
Requires dust
Fundamental physics
Gastrophysics

4. DLAs: Motivation for 21 cm Searches
Optical spectroscopic selection:
Requires background UV source
Redshift into atmospheric window (z > 1.65)
Pencil beam (single los, single cloud)
DLA studies:
Damped/saturated line
(very important for EOR studies!)
Optical identification of DLAs
Molecular absorption:
Extremely rare
Requires dust
Fundamental physics
Gastrophysics
HI 21 cm:
Dust!
Any z
Multiple los, clouds
(+ Einstein rings!)
21 cm HI:  =
(cf Radio Probes of Reionization, 2007)
Optically faint QSOs
All molecular absorbers show HI 21 cm absorption

5. Molecular Absorbers: The Usual Suspects
Four known at z > 0.2
(OH only excluded)
Large searches have produced no new objects
Molecular Absorption
Requires fortuitous alignment
Requires flat or inverted continuum
Detectability is independent of redshift
Gastrophysics
Provides only means to detect unexceptional ISM at z > 0
Precision Measurements
Can measure fundamental constants at high redshift

6. Molecular Absorption Systems at z > 0.2
PKS
z =
AGN in spiral B
z = B
z =
Lens (Einstein ring)
PKS
z =
Conjugate OH lines
No “main” lines
Wiklind & Combes 1997
Darling (2004)
Darling (in prep)
Wiklind & Combes 1996
Darling (in prep)
Kanekar et al 2003
Wiklind & Combes 1995
Kanekar et al 2003
Wiklind & Combes 1998
Darling (in prep)

7. Molecular Absorption Systems at z > 0.2
PKS
z =
AGN in spiral B
z = B
z =
Lens (Einstein ring)
PKS
z =
Conjugate OH lines
No “main” lines
Wiklind & Combes 1997
Darling (2004)
Darling (in prep)
Wiklind & Combes 1996
Darling (in prep)
Kanekar et al 2003
Wiklind & Combes 1995
Kanekar et al 2003
Wiklind & Combes 1998
Darling (in prep)

8. PKS 1413+135: OH and HI Absorption
OH satellite lines:
1612, 1720 MHz
(see also Kanekar et al. 2004)
Systematic offset from HI
Is the offset physical?
How to assess offsets?
13 km s-1

9. PKS : OH, HI & CO
HI, CO redshifts exceptionally well measured (< 10-6); systematics dominate
(Carilli et al. 1998)
OH: Systematic offset from HI, CO
OH-only measure consistent with zero
Δν/HI: systematics can account for all Δα/αo
HI: Darling 2004
OH: Darling 2004
CO: Wiklind & Combes 1997
(1×10-5 ~ 10 km s-1)

10. PKS : OH, HI & CO
HI, CO redshifts exceptionally well measured (< 10-6); systematics dominate
OH: Systematic offset from HI, CO
OH-only measure consistent with zero
Δν/HI: systematics can account for all Δα/αo

11. Intervening 21 cm Absorption: A “blind” survey at Green Bank
Goals:
Conduct a DLA search analogous to optical surveys (large ∆z).
Minimize DLA selection biases (z, dust).
Requires:
Large instantaneous z coverage
Good velocity resolution (∆v < 10 km s-1)
Sensitivity to all DLAs in short integrations
Bonus:
Simultaneous search for OH lines
Two Surveys:
continuum sources (S > 0.8 Jy) in 0.6 < z < 1.1 (Darling & Giovanelli)
flat-spectrum sources from z = 0 to z = zsys (Bolatto & Darling)

12. Green Bank Blind HI Survey:
Observe 200 MHz at 800 MHz with 6 kHz (~2 km s-1) resolution
λ /Δλ = 132,000
BW/λ = 0.25
Radio Freq
Interference
(RFI) is
problematic and
reduces z
coverage
Feed Resonance
RFI
z = 0.63
tl = 5.9 Gyr
tU = 7.8 Gyr
z = 1.1
tl = 8.1 Gyr
tU = 5.6 Gyr
2.2 Gyr, 1.2 Gpc

13. Green Bank Blind HI Survey
Chengalur, deBruyn, &
Narasimha 1999
Patnaik et al. 1994
Nair et al. 1993
Pure radio HI absorption detection!
Proof-of-concept for blind searches.
FWHM = 57 km s-1
τc = 0.22
NHI = 24.3 x 1018 (Ts/f) cm-2
PKS
Molecules?
Lens?

14. Green Bank Blind HI Survey
Chengalur, deBruyn, &
Narasimha 1999
Patnaik et al. 1994
Nair et al. 1993
BIMA (Bolatto)
Pure radio HI absorption detection!
Proof-of-concept for blind searches.
FWHM = 57 km s-1
τc = 0.22
NHI = 24.3 x 1018 (Ts/f) cm-2
PKS
Molecules?
Lens?

15. Intervening 21 cm Absorption: A “blind” survey at Green Bank
Goals:
Conduct a DLA search analogous to optical surveys (large ∆z).
Minimize DLA selection biases (z, dust).
Requires:
Large instantaneous z coverage
Good velocity resolution (∆v < 10 km s-1)
Sensitivity to all DLAs in short integrations
Proof of Concept:
Detection of at z = 0.78 (no a priori knowledge of this DLA)
Recovery of known absorbers
Expectations:
∆z ~ 150 (including RFI losses)
Estimate of ΩHI (but depends on Tspin)

16. Intrinsic 21 cm Absorption: CSOs
Compact Symmetric Objects:
Compact (< 1 kpc)
Symmetric (jets)
Post-Mergers
Inside-out virialization
(t ~ 108 yr, Perlman et al 2001)
Jet advance shows radio
source turn-on
Crossing time of
nucleus « jet lifetime
 nucleus at birth of RL AGN
Dust and gas still in cores
(yet to be expelled)
Peck & Taylor 2002

17. Intrinsic Absorption: Survey Expectations
Observe:
71 sources
0.5 < z < 4
CSOs
GPS sources
CSS sources
Expect:
Detect all DLAs in
1-2 hours
Bonus:
OH lines
 DLAs 

18. Intrinsic Absorption
Expected HI

19. Barycentric Frequency (MHz)
Intrinsic Absorption
Expected HI
PKS
z = 0.58
Flux Density (Jy)
Previous detection:
Carilli et al 1998
Barycentric Frequency (MHz)

20. Intrinsic Absorption: Survey Results (so far…)
z < 0.7 redetections

21. Intrinsic Absorption: Survey Results (so far…)
No new detections
Previous surveys have
30-50% detection rate
atz < 0.7
(Vermeulen et al 2003)
Sub-DLAs detectable
Adequate sensitivity
to z ~ 3, including
RFI losses
Work continues…
z < 0.7 redetections

22. H2CO: The Swiss Army Knife Molecule
Darling & Goldsmith (in prep)
NGC 2264
Galactic
Extragalactic
Gastrophysics
Galaxy Evolution
(Cosmology ?)

23. Darling & Goldsmith (in prep) Darling & Goldsmith (in prep)
Galactic H2CO
Dark Clouds:
- “Anomalous” H2CO absorption
(e.g. Palmer et al. 1969)
- Absorption in multiple cm lines
- No radio continuum source!
Barnard 227
Darling & Goldsmith (in prep)
NGC 2264
Darling & Goldsmith (in prep)

24. H2CO: The DASAR L ight A mplification by S timulated E mission of
R adiation
Inversion:
“Heating” of lines
Tx >> Tkin
Pump required:
Chemical, collisional, radiative
D arkness*
A mplification** by
S timulated
A bsorption of
R adiation
Townes et al (1953)
Anti-Inversion:
“Cooling” of lines
Tx < TCMB
Pump required:
Collisions with H2
*Not really dark.
**Not a true amplification.

25. Darling & Goldsmith (in prep) Darling & Goldsmith (in prep)
Galactic H2CO
Dark Clouds:
- “Anomalous” H2CO absorption
(e.g. Palmer et al. 1969)
- Absorption in multiple cm lines
- No radio continuum source!
Can H2CO be observed in other galaxies?
2. Can “anomalous” H2CO absorption be observed in galaxy-scale analogs of Dark Clouds?
Barnard 227
Darling & Goldsmith (in prep)
NGC 2264
Darling & Goldsmith (in prep)

26. Extragalactic H2CO Maser Emission in (U)LIRGs (OH Megamasers) Arp 220
Darling & Wiklind
Extragalactic H2CO
Maser Emission in (U)LIRGs
(OH Megamasers)
Arp 220
III Zw 35
Absorption in starbursts
(OH absorbers)
NGC 520
NGC 660
Absorption in dense clouds B
Biggs et al 2001

27. Ortho-H2CO Toward B0218+357 Previous Detections:
4.8 GHz ( ) line detected at Arecibo
Two gaussian components
app = ± 0.003
∆v = 12.6 ± 0.6 km s-1
Previous Detections:
14.5 GHz ( ) (VLA; Menten & Reid 1996)
150.5 GHz ( ) (IRAM; Wiklind & Combes)
Darling & Wiklind
Wiklind & Combes

28. H2CO Toward B : Summary
Similar to Galactic Dark Clouds (but scaled to CMB at z = 0.67)
Centimeter lines (4.8, 14.5 GHz) are anti-inverted
T4.8 ~ 2.3 K
T14.5 ~ 3.4 K (TCMB = 4.6 K)
Millimeter lines (150.5, GHz) have Tex ~ TCMB
N(ortho-H2CO) = 1.5  1013 cm-2
N(H2) = 1.5  cm-2
n(H2) = cm-3
Future Prospects:
Ortho:para at z > 0
Prediction: H2CO can be observed in absorption against CMB in extragalactic ISM
- How does T decrement scale with z?
- What is H2CO filling factor?

29. H2CO Absorption Against the CMB

30. H2CO: The DASAR The CMB is the ultimate illumination source:
Behind everything
Everywhere
Uniform on arcsec scales
H2CO absorption against the CMB offers an unrivaled probe of dense molecular gas, independent of redshift!

31. H2CO Against the CMB: Prospects
Step 1: Local Calibration
Survey local galaxies, from spirals to ULIRGs
Include sample with CO and HCN measurements
What is the filling factor on kpc scales?
What is the total H2CO mass? M(H2CO)  M(dense)
GBT: large survey in 14.5 and 4.8 GHz lines
(Darling, Mangum, Menten, & Henkel)
Step 2: Submm Galaxies
How does anti-inversion scale with redshift?
What is dense gas fraction?
VLA: deep integrations in line at z ~ (Darling & Baker)

32. Radio-Selected Damped Lyman  Systems
New Radio Facilities Allow Optical-Style Surveys
Intervening absorption, independent of dust
Proof of concept detection of DLA
∆z ~ 150
Intrinsic Absorption
Expect high detection rates
Sensitivie to DLAs to z = 4
OH search for free
Stimulated Absorption by H2CO (DASARs)
Uses CMB as illumination source
Traces gastrophysics in detail
Potentially very large pool of objects to observe
(still much foundational/calibration work to be done…)

33. The End

34. Conjugate OH: Anti-masing
Conjugate OH lines:
Selection rules:
ΔF = ± 1,0
Intra-ladder transitions overpopulate F = 2:
1720 emission
1612 absorption
Inter-ladder transitions overpopulate F = 1:
1720 absorption
1612 emission
1720
1612

35. Conjugate lines in NGC 253 Frayer, Seaquist & Frail (1998) Note:
Conjugate OH lines show changing structure along line of sight:
1720 emission  N(OH)/V < 1015 cm-2 km-1 s
 N(H2) < 1022 cm-2
1612 emission  N(OH)/V > 1015 cm-2 km-1 s
 N(H2) > 1022 cm-2
Note:
Conjugate lines weakly amplify background continuum
 Detectability follows rules of absorption, not emission

36. H2CO Absorption in Dark Clouds
“Anomalous absorption” in Galactic dark clouds (Palmer et al 1969)
 Tex < TCMB
2 cm lines also observed in absorption against CMB
“Anti-inversion” due to collisional pumping (Evans et al 1975)
cm line ratio proxy for n(H2)
2 mm emission observed in Galactic dark clouds (Evans & Kutner 1976)
 gastrophysics

37. H2CO: The Swiss Army Knife Molecule
Anti-inverted (cm) line ratios yield n(H2), nearly independent of Tkin
Line ratios between species give ortho:para ratio
H2CO formation channel (hot/cold; gas/dust)
Line ratios from different Ka ladders of a given species (ortho/para) yield Tkin
∆J = ±1 line ratios within a Ka ladder yield Trot

38. Extragalactic H2CO Maser Emission in (U)LIRGs (OH Megamasers) Arp 220
Baan, Guesten, &
Haschick (1986)
Extragalactic H2CO
Maser Emission in (U)LIRGs
(OH Megamasers)
Arp 220
III Zw 35
Absorption in starbursts
(OH absorbers)
NGC 520
NGC 660
Absorption in dense clouds B
Darling & Henkel

39. Extragalactic H2CO Maser Emission in (U)LIRGs (OH Megamasers) Arp 220
III Zw 35
Absorption in starbursts
(OH absorbers)
NGC 520
NGC 660
Absorption in dense clouds B
Darling & Henkel (in prep)
NGC 660, 8.4 GHz
Filho, Barthel, & Ho (2002)

40. Extragalactic H2CO Maser Emission in (U)LIRGs (OH Megamasers) Arp 220
III Zw 35
Absorption in starbursts
(OH absorbers)
NGC 520
NGC 660
Absorption in dense clouds B
Darling & Henkel (in prep)
NGC 660, 8.4 GHz
~375 km/s
~350 pc
Mencl = 1.4109 M
Filho, Barthel, & Ho (2002)

41. H2CO: A Planar Asymmetric Top Molecule
Wiklind & Combes

42. H2CO: Anti-Inversion in Centimeter Lines
Allow 4 excitation temps:
No physical solution with Tcm > TCMB
No solution with Tmm= Tcm
No solution with Tmm= T4.8
If T14.5 = T4.8 then all lines have Tex < TCMB
If Tmm ≥ TCMB then TCMB > T14.5 > T4.8

43. Extended Illumination: PKS 1830-211
Chengalur, deBruyn, &
Narasimha 1999
Patnaik et al. 1994
Nair et al. 1993
PKS :
- Einstein ring at z = 0.89
- HI and OH absorption
- CO, HCN, HCO+,… absorption
- Moleculear isotope absorption
- H2CO absorption
PKS
Menten et al. 1999
300 km/s
Darling (in prep)

44. Scaling Relations Detection of cm H2CO Lines vs z Depends on:
(Anti) Inversion vs z
How does Tcm - TCMB scale vs z?
(TCMB = 2.73 (1+z) K)
Filling factor on kpc scales
Filling factor vs z
Angular size vs z

45. Scaling Relations Detection of cm H2CO Lines vs z Depends on:
(Anti) Inversion vs z
How does Tcm - TCMB scale vs z?
Tcm - TCMB  (1+z)
Filling factor on kpc scales
Filling factor vs z
Angular size vs z

46. Scaling Relations Detection of cm H2CO Lines vs z Depends on:
(Anti) Inversion vs z
How does Tcm - TCMB scale vs z?
Tcm - TCMB  (1+z)
Filling factor on kpc scales
Filling factor vs z
Angular size vs z

47. Scaling Relations  CMB power scales as beam arcmin:arcsec  3600:1
Detection of cm H2CO Lines vs z Depends on:
(Anti) Inversion vs z
How does Tcm - TCMB scale vs z?
Tcm - TCMB  (1+z)
Filling factor on kpc scales
Filling factor vs z
Angular size vs z
- CMB power in small beams
Rayleigh-Jeans Law
 CMB power scales as beam
arcmin:arcsec  3600:1
~100 mJy  ~30 µJy

48. H2CO Against the CMB: Prospects
The Future:
- Molecule of choice for studies of star formation, molecular gas from present day to arbitrary redshift
EVLA
ALMA
High Sensitivity Array
Ortho:Para H2CO gives astrochemistry channel
(dust vs gas, hot vs cold)
H2CO mm + cm lines yield gastrophysics
Tkin n(H2) Tx TCMB
No redshift limit to detection (in fact, angular size grows at high z)

49. H2CO as z-Machine If H2CO can be observed against the CMB,
Anti-inversion obviates need for chance alignments
Unique probe of gastrophysics of dense molecular ISM
Much foundational work yet to be done…
Scaling relations
Filling factor on kpc scales
Total H2CO mass in galaxies: M(H2CO)  M(dense)
Regardless, H2CO should be observable with ALMA:
Absorption and emission
Similar abundance, line luminosity to HCN (~10%)
Ortho:para H2CO at z > 0
Line Tex floor set by CMB, scales with z

50. Pathologies as Probes Masers provide exceptional Tb
Precision positions (H2O in NGC 4258)
Probes of intervening gas (scintillation)
Signposts at cosmological distances
Tunneling
NH3 is a maser and molecular ISM thermometer
Conjugate Lines
Local H2 density indicator
Probe of fundamental physical constants
Stimulated Absorption (DASARs)
Uses CMB as illumination source
Traces gastrophysics in detail
Potentially very large pool of objects to observe
(still much foundational/calibration work to be done…)